1. Field of the Invention
The present invention relates to audio frequency signal transmission systems in general and in particular to an audio frequency signal transmission line and a method of making the same comprising means for reducing attenuation and phase shift of low frequency components of audio signals relative to high frequency components of audio signals which are transmitted between a source of audio signals and a load by means of a pair of electrical conductors.
2. Description of the Prior Art
An audio signal transmission system of the type to which the present invention relates comprises a source of audio signals coupled to a load by means of a first and a second electrical conductor. For example, the audio signal source may comprise an audio signal preamplifier, a power amplifier, a transducer or the like, for providing signals in the audio frequency spectrum; the load may comprise a preamplifier, a power amplifier, a speaker or other load depending on the source; and the first and second electrical conductors may comprise a pair of parallel untwisted insulated wires, a pair of twisted insulated wires, or a coaxial cable.
The audio frequency spectrum is generally considered to encompass a frequency range of from 20 Hz to 20,000 Hz.
Various means have been proposed for improving the transmission of audio signals between amplifiers, between an amplifier and a speaker, and between other audio signal sources and a load. For example, in U.S. Pat. No. 4,177,431, there is provided an interface circuit for suppressing noise between a power amplifier and a speaker coupled thereto by a "super speaker" cable. The interface circuit comprises a series coupled capacitor and resistor which is coupled across the input terminals of the speaker at the output end of the super speaker cable for providing an interface between the super speaker cable, which has a low characteristic impedance, e.g. 9 ohms, and the speaker, which also has a low characteristic impedance of 4-6 ohms over the audio frequency range. In applicant's co-pending U.S. patent application entitled "Audio Signal Transmission System With Noise Suppression Network", Ser. No. 07/354,517, filed May 18, 1989, there is disclosed an audible noise suppression interface circuit comprising an RC network which is coupled across the output terminals of an audio signal source at the input end of an audio signal transmission line. This interface circuit suppresses noise in the audio spectrum which would otherwise be generated by either the transmission line or load becoming momentarily open-circuited in response to certain low frequency signals. In applicant's U.S. Pat. No. 4,718,100, there is disclosed a method and apparatus for reducing phase shift between high and low frequency components of audio signals transmitted between a signal source and a load. In applicant's patent the method and apparatus disclosed comprises one or more electrical conductors for providing additional distributed capacitance along one or both of two transmission lines coupling a source of audio signals to a load. In each case, one end of the electrical conductor providing the additional distributed capacitance is connected to either a terminal of the source or a terminal of the load while the other end of the electrical conductor is left unterminated.